The overall objective is to identify domains of the large T antigen of SV40 that modulate the transformation of cells in culture and tumorigenicity. More specifically, we will identify those portions of this dominant viral oncogene product that confer individual characteristics of the transformed cell phenotype and those that are dispensible to the processes. We have already shown that at least some of the transformed cell properties are separable genetically. By localizing immortalizing capacity, ability to form dense foci, ability to form colonies in semisolid medium and tumorigenicity to distinct portions of T antigen it should be possible to predict those functions of the protein that cooperate in fully transforming specific rodent cells in culture and in producing tumors. Particular interest will placed on the role of T antigen in the plasma membrane in the processes of transformation and tumorigenesis. Both detection and immune-selection methods for the isolation and analysis of mutants that produce T antigens that have lost the ability to associate normally with the surface of cells are proposed. In addition, the association between functions marked by the hr-t mutant in polyoma virus and a mutant of SV40 with a similar phenotype will be probed. Finally, three specific regions of the T antigen coding sequence, a possible immortalizing domain, a region possibly associated with aberrent expression of surface T antigen, and the nucleotides surrounding the lesion in the hr-t-like mutant of SV40, will be mutagenized exhaustively in order to expand representation of these areas by mutants whose characterization will assess the impact of specific functions on lytic growth and transformation by SV40.